This invention pertains to an improved process for preparing essentially solvent-free aqueous epoxy resin dispersions or low solvent emulsions useful in the preparation of water-dilutable, heat-curable coating compositions, and to the dispersions or emulsions made by said process.
It is well known in the art that many synthetic resins can be prepared by emulsion polymerization and that it is not difficult to prepare stable aqueous dispersions of said resins by addition of the solid resin and an appropriate dispersing agent to water with stirring. However for polymers such as the epoxy resins, which cannot be prepared by emulsion polymerization, aqueous dispersions must be made by dispersing the bulk polymer in water. Such dispersions are generally quite unstable separating in only a short time as from a few hours to a few days. These dispersions of bulk epoxy resins also generally exhibit poor film forming properties. Both of these disadvantages of poor dispersion stability and poor film properties are largely due to the large particle size of the resin. The particle size of the resin in the previously formed dispersions of bulk polymers have been of the order of 50 microns or larger.
The preparation of polyepoxide dispersion coating compositions is disclosed in U.S. Pat. No. 3,772,228, wherein a one-package, thermoset coating composition is prepared by grinding and dispersing a solid, friable polyepoxide, a solid, friable epoxy curing agent (such as a polyanhydride) and optionally an epoxy curing accelerator in a liquid which is a non-solvent for the various components. Aliphatic hydrocarbons are preferred. These are epoxy dispersions, but they are not aqueous and possess the inherent hazards involved in the use of hydrocarbon solvents.
The preparation of organic solvent free stable aqueous dispersions of epoxy resins of relatively low molecular weight (200 to 4,000, preferably 240 to 1,300) having particle size of less than about 10 microns using cationic, anionic and nonionic dispersing agents is disclosed in U.S. Pat. No. 3,879,324. Cationic dispersing agents are preferred. The process described to prepare said epoxy dispersions involved heating the epoxy resin to the fluid state, mixing with water and the dispersing agent at the elevated temperature, followed by passing the mixture through a colloid mill. Only those epoxy resins melting below 100.degree. C., the boiling point of water, can be dispersed by this process accounting for the molecular weight limitation noted. This limitation is a severe one eliminating many useful epoxy resin systems of high molecular weight which can be employed in the instant process.
The preparation of stable aqueous polyurethane elastomer dispersions in the absence of added surfactants is described in U.S. Pat. No. 3,905,929 by the process of dissolving the polyurethane in an organic solvent, miscible with water and inert toward isocyanate groups such as acetone or methyl ethyl ketone, adding water to the stirred solution and removing the organic solvent by distillation. The preparation of stable aqueous polyurethaneamide dispersions, also in the absence of added surfactants, by the same procedure is taught in U.S. Pat. No. 3,935,146.
The preparation of aqueous dispersions of solid epoxy resins is described in German Offen. No. 2,424,887 by the process of passing to a high shear mixer a first stream consisting of water and certain dispersing agents and a second stream containing a molten solid resin or a solution of a solid resin in certain organic solvents, mixing at high shear to give the stated degree of dispersion and removing the organic solvent if present by vacuum distillation.
The dispersing agents useful in the process of German Offen. No. 2,424,887 are primarily synthetic materials derived from cellulose, such as guargums, and polyvinyl alcohols which can have terminal or pendant alkyl ether, arylalkyl ether, phenoxy, amide or amine groups. Examples of dispersing agents useful in this process include polyvinyl alcohol, guargum and some nonylphenoxypoly(ethyleneoxy)ethanol materials.
The instant invention is distinguished from the above process in that, among the myriad of possible dispersing agents which might be employed in the general process described in German Offen. No. 2,424,887, one class of dispersing agents, namely the poly(ethylene oxide) esters of fatty acids, gives far superior performance both in the improved process itself, but also in the properties of the coatings and finishes made using the instant compositions.
Additionally, German Offen. No. 2,424,887 describes the use of solid epoxy resins having a molecular weight under the lower 4,000 molecular weight limit of the instant process. The surfactants described by this reference are relatively ineffectual in the instant process with higher molecular weight epoxy resins. Criticality exists between the instant class of nonionic surfactants required to prepare in a practical way dispersions or low solvent emulsions of high molecular weight epoxy resins as described in the instant process.